This invention generally relates to spread spectrum time division duplex (TDD) communication systems. More particularly, the present invention relates to a system and method for controlling downlink transmission power within TDD communication systems.
Spread spectrum TDD systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective chip code sequences (codes). Referring to FIG. 1, TDD systems use repeating frames 34 divided into a number of time slots 371-37n, such as fifteen time slots. In such systems, a communication is sent in a selected time slot out of the plurality of time slots 371-37n using selected codes. Accordingly, one frame 34 is capable of carrying multiple communications distinguished by both time slot and code. The combination of a single code in a single time slot is referred to as a physical channel. Based on the bandwidth required to support a communication, one or multiple physical channels are assigned to that communication.
Most TDD systems adaptively control transmission power levels. In a TDD system, many communications may share the same time slot and spectrum. While user equipment (UE) 22 is receiving a downlink transmission from a base station, all the other communications using the same time slot and spectrum cause interference to the specific communication. Increasing the transmission power level of one communication degrades the signal quality of all other communications within that time slot and spectrum. However, reducing the transmission power level too far results in undesirable signal to noise ratios (SNRs) and bit error rates (BERs) at the receivers. To maintain both the signal quality of communications and low transmission power levels, transmission power control is used.
The standard approach to TDD downlink power control is a combination of inner and outer loop control. In this standard solution, the UE transmits physical layer transmit power control (TPC) commands to adjust the base station transmission power. A base station sends a transmission to a particular UE. Upon receipt, the UE measures the signal interference ratio (SIR) in all time slots and compares this measured value to a SIRTARGET. This SIRTARGET is generated from the Block Error Rate (BLER) signaled from the base station.
As a result of the comparison of the measured SIR value with the SIRTARGET, the UE transmits a TPC command to the base station. The standard approach provides for a TPC command per coded composite transport channel (CCTrCH). The CCTrCH is a physical channel which comprises the combined units of data for transmission over the radio interface to and from the UE or base station. This TPC command indicates to the base station to adjust the transmission power level of the downlink communication. The base station, which is set at an initial transmission power level, receives the TPC command and adjusts the transmit power level in all time slots associated with the CCTrCH in unison.
This approach to TDD downlink power control works well as long as the interference in each time slot is the same. Unfortunately, in most cases, the interference in each time slot is different. A small difference may be acceptable due to the averaging effect of the interleaving, but larger differences cause degradation due to thresholding effects in the receiver. This requires the receiver to have a wider dynamic range and unnecessarily high transmit power in some time slots. An adjustment made to the base station SIRTARGET for all time slots based on the error value may create an unbalanced increase or decrease of the power level. In other words, those time slots where the power level was lower than the initial value of the base station will be adjusted even lower when the calculated error value was higher than the SIRTARGET. These low level power time slots may then be eliminated from detection, thereby the transmission will be degraded. The same is true for those time slots in which the power level was higher than the SIRTARGET of the base station. When the detected error rate is lower than the SIRTARGET, the higher power level time slots will be increased, thereby creating interference with other channels on the system.
Accordingly, there is a need to have an approach to TDD downlink power control which adjusts the power level of each slot individually.